Interdiffusion resistant Fe--Ni alloys having improved glass sealing

ABSTRACT

The present invention relates to an iron-nickel alloy containing from about 30% to about 60% nickel, from about 0.001% to about 0.15% nitrogen, at least one element selected from the group consisting of from about 1% to about 10% molybdenum and from about 0.001% to about 2% aluminum and the balance essentially iron. The alloys demonstrate improved resistance to intermetallic compound formation, improved glass to metal sealing properties, and improved wirebonding performance. The alloys of the present invention have particular utility as a lead frame material for semiconductor packages.

The present invention relates to iron-nickel alloys having particularutility in electronic and electrical applications. The iron-nickelalloys contain nitrogen and at least one element selected from the groupconsisting of molybdenum and aluminum.

Iron-nickel alloys and iron-nickel-molybdenum alloys in particular arewell known magnetic alloys. These alloys are characterized by suchdesirable magnetic properties as high initial permeability, highresistivity, good consistency of permeability, and good magneticstability. U.S. Pat. Nos. 1,757,178 to Elmer, 2,407,234 to Guthrie etal., 2,891,883 to Howe, 3,024,142 to Parkin, 4,082,580 to Pfeifer etal., and 4,536,229 to Jin et al. illustrate some of these iron-nickeland iron-nickel-molybdenum magnetic alloys.

Iron alloys including iron-nickel alloys have also found use inelectronic applications. For example, lead frames in CERDIPs and pins inTO cans have been fabricated from iron-nickel alloys. The use of thesealloys, however, has not been without serious problems. In semiconductordevices, the iron-nickel alloy lead frame is typically joined to anintegrated circuit chip by a number of aluminum and/or aluminum alloylead wires. The junctions between the iron alloy lead frame and thealuminum lead wires are typically sites where brittle intermetalliccompounds are formed as a result of the strong tendency forinterdiffusion between iron and aluminum upon exposure to heat, such asthe thermal treatment(s) associated with packaging the semiconductordevice.

This intermetallics problem is generally dealt with by forming a thinaluminum stripe on one or more surfaces of the iron alloy lead frame.The aluminum stripe may be a thin coating applied to the surface or aninlay. In some cases, the stripe comprises a coating over an entire leadframe surface. The lead wires are then bonded to the aluminum stripe sothat an aluminum/aluminum couple is formed. U.S. Pat. No. 3,559,285 toKauffman illustrates an iron alloy lead frame material having an inlaidaluminum stripe.

Another problem associated with the use of iron alloys as lead framematerials has to do with glass sealing. Many semiconductor integratedcircuit chips are packaged in a ceramic package known as a CERDIP. Toprovide a hermetic package structure, the lead frame is bonded to one ormore layers of a ceramic material by a sealing glass material such asthose having a lead oxide base. Iron alloys such as Fe-42Ni do not bondwell to sealing glass materials. Again, an aluminum stripe on or in thelead frame material is used to overcome this bonding problem sincealuminum typically bonds well to sealing glasses.

Manufacturers of semiconductor devices and/or packages have expressed adesire for iron alloys that can be used as lead frame materials withouthaving to coat or stripe the lead frames. By avoiding coating and/orstriping of the lead frame, the manufacturing costs can be reduced.Desirable alloys are those characterized by improved resistance tointermetallic compound formation and improved bonding ability to sealingglass materials.

Accordingly, it is an object of the present invention to provide aniron-nickel alloy having improved resistance to intermetallic compoundformation.

It is a further object of the present invention to provide an alloy asabove having improved glass adhesion properties.

It is a further object of the present invention to provide an alloy asabove having improved wirebonding characteristics.

It is a further object of the present invention to provide iron-nickelalloys as above suitable for use as lead frame materials insemiconductor devices.

These and other objects and advantages will become more apparent fromthe following description and drawings wherein like reference numeralsdepict like elements.

The present invention achieves the aforementioned objects by makingadditions of nitrogen and at least one element selected from the groupconsisting of molybdenum and aluminum to iron-nickel alloys. Theaddition of nitrogen and molybdenum with or without aluminum toiron-nickel alloys has been found to improve the resistance of thealloys to the formation of intermetallic compounds, particularly duringthe heat treatments associated with typical semiconductor manufacturingand/or packaging techniques. Further, alloys containing these additionshave been found to have improved bond adhesive with glass sealingmaterials and to be readily bondable to aluminum or aluminum alloy leadwires using standard commercial ultrasonic bonding techniques.

Alloys in accordance with the present invention contain from about 30%to about 60% nickel, from about 0.001% to about 0.15% nitrogen, at leastone element selected from the group consisting of about 1% to about 10%molybdenum and from about 0.001% to about 2% aluminum, and the balanceessentially iron. The alloys are characterized by having a single phaseand a face centered cubic metallurgical structure. The alloys alsopreferably have an iron:nickel ratio in the range of about 1.1:1 toabout 1.6:1. Preferred alloys contain from about 39% to about 55%nickel, from about 0.005% to about 0.05% nitrogen, at least one elementselected from the group consisting of about 1% to about 4% molybdenumand about 0.3% to about 1.05 aluminum, and the balance essentially iron.

The Figure is a cross sectional view of a semiconductor package.

The present invention relates to iron-nickel alloys having improvedresistance to intermetallic compound formation and/or improved adhesionto glass sealing materials. These alloys have particular utility inelectrical and electronic applications. For example, the alloys may beused for lead frames or similar component sin semiconductor packages.Alternatively, they can be used for pins in TO cans, glass-to-metalpower feed throughs or other similar applications.

Referring now to the Figure, a typical semiconductor package 10 isillustrated. The package 10 comprises a ceramic base 12, a first glasslayer 16 and a number of leads 18 from a lead frame bonded to theceramic base 12 by the glass layer 16. A semiconductor device 20 ismounted to the base 12 by either a die attache pad or a layer of goldcontaining material 14. The gold containing material is used in manymodern packages to permit formation of a gold-silicon eutectic bondbetween the layer 14 and the chip 20. The gold containing layer 14 maycomprise either a gold plating or a gold paste fired to the ceramic base12. The device 20 is connected to the leads 18 by a number of lead wires22. Generally, the lead wires are formed from aluminum or an aluminumalloy such as Al--1Si. A second glass layer 24 is positioned over theleads 18 of the lead frame assembly. To complete the package, a cover 26formed from a ceramic or metallic material is placed over the glasslayer 24.

In some packages, the glass layer 24 and the cover 26 each have acentral aperture or window to permit the device 20 to be bonded to thepad or layer 14 and the wire connections to be made after the glasslayer 24 and/or the cover 26 have been fused to the glass layer 16. Acap not shown is provided to close the window after the device has beenpositioned on the pad and the wire connections are made. The cap may beformed from a ceramic material, a metallic material such as gold platedKovar, or a glass material.

To fabricate the semiconductor package illustrated in the Figure, thedie attach pad or gold containing layer 14 is first bonded to or platedonto the ceramic base 12. The glass layer 16 is then screen printed onthe ceramic base and air fired, leaving an aperture or window 23 forconnecting the semiconductor device 20 to the pad or layer 14. The leadframe with the leads 18 is then positioned on the glass layer 16 andfused into placed. Preferably, the second glass layer 24 is joined tothe cover 26 before being fused to the layer 16. Thereafter, the glasslayer 24 is fused to the glass layer 16 to form a hermetic packagestructure. Prior to this, however, the device 20 is attached to the pador layer 14 and the wire interconnections are made between the device 20and the leads 18. Typically, the wire interconnects are made usingeither a thermocompression, ultrasonic or thermosonic bonder in anambient atmosphere. This wire bonding operation often results in surfaceoxides being formed on the leads 18.

Typical ceramic materials used in packaging integrated circuit chips orsemiconductor devices include aluminum oxide and beryllium oxide. Theglass layers 16 and 24 in these packages may be formed from any suitableglass material such as an 85% lead oxide--15% boric acid composition.

It should be recognized that the package shown in the Figure and thediscussion attendant thereto are meant to be illustrative and are notmeant to limit the scope of the invention. The alloys of the presentinvention may be used in conjunction with a wide variety of packageconstructions and materials.

As previously discussed, the lead 18 and the lead frames in many modernpackages are formed from iron alloys such as Fe--42Ni. The use of thesealloys as lead frame materials has engendered several significantproblems. These problems include: (1) the formation of intermetalliccompounds at the junctions between the leads 18 and the lead wires 22,particularly when the lead wires 22 are formed from aluminum or analuminum alloy; and (2) poor adhesion between the leads 18 and the glasslayers 16 and 24. The intermetallic compound problem is significantbecause it adversely affects the quality of the wire interconnections.Many semiconductor devices fail because the wire interconnections becomebrittle and break. The poor adhesion problem is important because it canlead to poor package hermetically characteristics and breakage of thepackage. Attempts to overcome these problems have included coatingand/or striping the leads 18 of the lead frames with materials such asaluminum, gold, and alloys thereof. Of course, coating and/or stripingincreases the costs associated with the manufacture of the package.

In accordance with the present invention, these problems are overcome byusing for the leads and the lead frame an iron alloy which exhibitsimproved resistance to intermetallic compound formation and/or improvedadhesion to glass sealing materials. Alloys of the present inventioninclude those having a composition consisting essentially of from about30% to about 60% nickel, from about 0.001% to about 0.15% nitrogen, atleast one element selected from the group consistion of about 1% toabout 10% molybdenum and from about, 0.001% to about 2% aluminum, andthe balance essentially iron. Preferably, the iron: nickel ratio in thealloy is in a range from about 1.1:1 to about 1.6:1.

Nitrogen has been found to be a particularly beneficial addition becauseit significantly reduces the formation of undesirable iron-aluminumintermetallic compounds. The molybdenum and aluminum additions arebeneficial in that they improve the glass adhesion properties andwirebonding performance of the alloys. Molybdenum is also useful in thatit increases the nitrogen solubility and/or diffusivity in theiron-nickel alloys of the present invention and is believed to interactwith surface oxides in a way that helps to improve bonding between thealloys of the present invention and glass sealing materials.

Particularly useful embodiments include: (a) an alloy consistingessentially of from about 39% to about 55% nickel, from about 0.005 % toabout 0.05% nitrogen, from about 1% to about 4% molybdenum and thebalance essentially iron; and (b) an alloy consisting essentially offrom about 39% to about 55% nickel, from about 0.005% to about 0.05%nitrogen, from about 0.3% to about 1.0% aluminum and the balanceessentially iron. Alloy (a) is particularly useful when the material issubjected to temperatures high enough to form intermetallic compounds.Alloy (b) is particularly useful when the temperatures are not highenough to form intermetallic compounds. If desired, an aluminum additionin the range from about 0.3% to about 1.0% may be made to alloy (a) tofurther improve the glass sealing properties of the alloy.

Metallurgically, the alloys of the present invention are characterizedby a face-centered cubic structure. In addition, the alloys are singlephase alloys.

When molybdenum is to be added to the iron-nickel alloys and theiron-nickel alloy is to be bonded to a pure aluminum component, theminimum molybdenum content for the alloy may be defined by using thefollowing equation:

    %Mo ≧(86±5-%Ni)/(7.25±1.25)                   (1)

For certain molybdenum embodiment alloys of the present invention, it isalso possible to determine a minimum nitrogen content for the alloy. Forexample, alloys containing nickel in the range from about 39% to about47% and molybdenum in the range from 1% to about 5.1%, it is preferredto make a minimum nitrogen addition in accordance with the followingequation:

    %N=(0.0001)×(341-7.5Ni+7.4Mo)                        (2)

If desired, the alloys of the present invention may also containmanganese in an effective amount up to about 1.0% and/or magnesium in aneffective amount up to about 0.1%. The manganese and/or magnesiumadditions are believed to be particularly useful in reducing the effectsof sulfur impurities during casting.

For improved intermetallic compound suppression properties, siliconshould not be present in the alloy. This is because silicon diminishesthe effect of the nitrogen on intermetallics by combining with thenitrogen to form silicon nitride. Conventional mill impurities may betolerated in the alloys of the present invention but should be kept atimpurity levels.

As used herein, the foregoing alloy composition percentages are weightpercentages.

The alloys of the present invention may be cast in any desired manner,including but not limited to Durville casting, continuous casting anddirect chill casting, into an ingot or strip form. After casting, thealloy may be hot worked such as by hot rolling and/or cold worked suchas by cold rolling with at least one interanneal. Preferably, the alloysare cold worked for strength without prior hot working. When cold workedfor strength, a reduction in the range of about 20% to about 40% istaken using standard cold rolling techniques.

The interanneals may be performed in a 96%N₂ --4%H₂ atmosphere at atemperature in the range of about 800° C. to about 1000° C. for a timein the range of about 1 hour to about 24 hours. Preferably, theinteranneals are performed in the aforesaid atmosphere at a temperaturein the range of from about 900° C. to about 975° C. for a time period inthe range of from about 2 hours to about 8 hours. If desired, theinteranneals may be used to charge the alloy with nitrogen.Alternatively, the nitrogen addition could be made by blowing nitrogengas into a melt containing iron, nickel, molybdenum and/or aluminum. Anysuitable technique known in the art such as using a blowing lance may beemployed to add the nitrogen to the melt. In yet another alternativeapproach, the alloy may be charged with nitrogen by annealing the castingot or strip in a cracked ammonia atmosphere.

After processing, the iron-nickel strip material may be fabricated intoa desired product such as a lead frame in accordance with any standardfabrication technique known in the art.

To demonstrate the improvements of the present invention, the followingexamples were performed.

EXAMPLE I

To demonstrate the improved resistance to intermetallic compoundformation possessed by the alloys of the present invention, a series ofalloys having the composition shown in Table I were prepared. The alloyswere charged with nitrogen by heating the alloys in a 96%N₂ --4%H₂atmosphere at 950 ° C.

                  TABLE I                                                         ______________________________________                                              Ni        Mo      Al       N     Fe                                     Alloy (wt %)    (wt %)  (wt %)   (wt %)                                                                              (wt %)                                 ______________________________________                                        A     42        --      --       .01   bal.                                   B     42        0.7-6   --       --    bal.                                   C     42        2.8-9.1 --       .01   bal.                                   D     41        --      0.7-2.1  .01   bal.                                   E     40        2.5     0.8-1.8  .01   bal.                                   ______________________________________                                    

A 2" ×4" coupon of each alloy was bonded to an aluminum alloy coupon.The aluminum alloy contained 1.25%Si. Bonding was done using thePOSIT-BOND® bonding technique in which the coupons were subjected to a50% cold working reduction. The coupons were mechanically cleaned anddegreased prior to bonding. The bonded coupons were exposed to 500° C.temperatures for 16 hours. The thickness of the intermetallic compoundsformed during this heat treatment was measured. Table II reports theresults of these measurements.

                  TABLE II                                                        ______________________________________                                                   Intermetallic Thickness                                            Alloy      (μm)                                                            ______________________________________                                        A          12                                                                 B          11-12                                                              C           2                                                                 D          11                                                                 E           2                                                                 ______________________________________                                    

The data in Table Ii shows that the thickness of the intermetalliccompounds formed between the Fe--Ni and Al--Si components was retardedby the addition of nitrogen and molybdenum with or without aluminum tothe iron-nickel alloys. This i evidenced by the results obtained usingAlloys S and E.

In order to manufacturers to dispense with using aluminum-coated or-striped iron-nickel alloy components in semiconductor packages, thesubstitute iron-nickel alloys should demonstrate equivalent orsubstantially equivalent glass sealing properties. The following exampleillustrates the improvements in glass sealing properties which can beobtained with the alloys of the present invention.

EXAMPLE II

A series of iron-nickel alloys having the compositions shown in TableIII were prepared. For comparison purposes, an aluminum coated Fe--42Nialloy sample also was prepared. Coupons of each of the alloys werebonded to a commercially available aluminum oxide CERDIP material so asto form a glass-metal-glass sandwich. Prior to bonding the iron alloycoupons were degreased.

After bonding the glass-metal-glass sandwiches were subjected to astandard Torque Test used in conjunction with integrated circuitpackages. In this test, the samples were twisted until they broke. TableIII reports the results of this test.

                  TABLE III                                                       ______________________________________                                        Alloy             Torque Strength (in-lb)                                     ______________________________________                                        Al-coated Fe--42Ni                                                                              74-82                                                       Fe--42Ni--0.0027N   0                                                         Fe--40Ni--2.8Mo--.0058N                                                                         >50                                                         Fe--41Ni--2.7Mo--0.8Al--.01N                                                                    >50                                                         Fe--42Ni--0.7Al--.01N                                                                           >50                                                         ______________________________________                                    

As can be seen from Table III, those alloys formed in accordance withthe present invention demonstrated torque strengths comparable to thoseachieved by coating Fe--42Ni with aluminum. Table III also shows thatmerely adding nitrogen to Fe--42Ni does not improve the glass sealingproperties of the alloy. The improvements in glass sealing adhesionprovided by the alloys of the present invention is believed to be theresult of obtaining a chemical bond between the glass and metal insteadof the squeeze mechanical type bond found with prior art materials.

In order to dispense with aluminum coating/striping of Fe--42Ni leadframes, it is also necessary to directly wire bond aluminum or aluminumalloy lead wires to a bare alloy lead frame and obtain failure loadscomparable to those obtained using the aluminum coated alloys. Thefollowing example illustrates the improvements in wirebonding obtainedwith the alloys of the present invention.

EXAMPLE III

A series of iron-nickel substrates having the compositions reported inTable IV were prepared. To simulate typical wire bonding operations, thesubstrates were wire bonded to a 1.25mil wire formed from an Al--1%Sialloy using a Kulicke & Soffa Model #484-8 wire bonder. All substrateswere subjected to an air frame sink treatment for 8 minutes at a peaktemperature of 493° C. Table IV reports the results of the average loadrequired to break the connection between the wire and the varioussubstrates.

                  TABLE IV                                                        ______________________________________                                                          Average Wire Pull Load                                      Substrate         (gm)                                                        ______________________________________                                        Al-striped Fe--42Ni                                                                             12                                                          Bare Fe--42Ni      9                                                          Fe--40Ni--2.8Mo--.01N                                                                           10                                                          Fe--42Ni--0.7Al--.01N                                                                           10                                                          Fe--41Ni--2.1Al--.01N                                                                            9                                                          Fe--41Ni--2.6Mo--0.45Al--.01N                                                                   12                                                          Fe--42Ni--2.5Mo--1.8Al--.01N                                                                    10                                                          ______________________________________                                    

As can be seen from Table IV, substrates formed from the alloys of thepresent invention generally exhibited improved wire bonding performanceas compared to bare iron-nickel alloy substrates.

While the alloys of the present invention have particular utility inelectronic and electrical applications, they also have utility in otherapplications where increased resistance to intermetallic compoundformation and/or improved glass adhesion properties are needed. Forexample, the alloys of the present invention may be used in compositestructures such as iron-nickel/glass composites and iron-nickelalloy/aluminum or aluminum alloy composites.

The patents set forth in the specification are intended to beincorporated by reference herein.

It is apparent that there has been provided in accordance with thisinvention interdiffusion resistant iron-nickel alloys having improvedglass sealing which fully satisfy the objects, means, and advantages setforth hereinbefore. While the invention has been described incombination with specific embodiments thereof, it is evident that manyalternatives, modifications, and variations will be apparent to thoseskilled int he art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives modifications, andvariations as fall within the spirit and broad scope of the appendedclaims.

What is claimed:
 1. A silicon free iron-nickel alloy for use inelectrical and electronic applications having improved resistance tointermetallic compound formation, said alloy consisting essentially offrom about 30% to about 60% nickel, from about 0.005% to about 0.15%nitrogen, at least one element selected from the group consisting offrom about 1% to about 105 molybdenum and from about 0.001% to about 2%aluminum, and the balance iron.
 2. The alloy of claim 1 wherein said atleast one element comprises molybdenum, said molybdenum increasing thenitrogen solubility and/or diffusivity of the alloy and being present inan amount in accordance with the following equation:
 3. The alloy ofclaim 1 wherein said at least one element consists of from about 1% toabout 4% molybdenum.
 4. The alloy of claim 1 further comprising:saidalloy having a single phase.
 5. The alloy of claim 1 furthercomprising:said alloy having a face centered cubic metallurgicalstructure.
 6. The alloy of claim 1 further consisting essentially ofsaid nitrogen being present in an amount from about 0.005% to about0.05%.
 7. The alloy of claim 1 wherein the ratio of iron to nickel insaid alloy is in the range of about 1.1:1 to about 1.6:1.
 8. The alloyof claim 1 further consisting essentially of at least one additionalelement selected from the group consisting of from an effective amountup to about 1.05 manganese and from an effective amount up to about 0.1%magnesium for reducing the effects of sulfur impurities during casting.9. The alloy of claim 3 further consisting essentially of said nitrogenbeing present in an amount from about 0.005% to about 0.05%.
 10. Asilicon free iron-nickel alloy for use in electrical and electronicapplications having improved resistance to intermetallic compoundformation, said alloy consisting essentially of from about 30% to about60% nickel, from about 0.005% to about 0.15% nitrogen, from about 0.3%to about 1.05 aluminum, and the balance iron.
 11. A silicon freeiron-nickel alloy for use in electrical and electronic applicationshaving improved resistance to intermetallic compound formation, saidalloy consisting essentially of from about 30% to about 60% nickel, fromabout 0.005% to about 0.15% nitrogen, from about 1% to about 4%molybdenum, from about 0.3% to about 1.0% aluminum, and the balanceiron.
 12. The alloy of claim 11 further consisting essentially of:theratio of iron to nickel in said alloy being in the range of about 1.1:1to about 1.6:1.
 13. The alloy of claim 11 further consisting essentiallyof at least one additional element selected from the group consisting offrom an effective amount up to about 1.0% manganese and from aneffective amount up to about 0.1% magnesium for reducing the effects ofsulfur impurities during casting.
 14. The alloy of claim 10 furtherconsisting essentially of:said nitrogen being present in an amount fromabout 0.005% to about 0.5%.
 15. The alloy of claim 14 further consistingessentially of:the ratio of iron to nickel in said alloy being in therange of from about 1.1:1 to about 1.6:1.
 16. The alloy claim 1 furtherconsisting essentially of said nickel being present in an amount fromabout 39% to about 55%.
 17. The alloy of claim 1 wherein said alloyconsists essentially of from about 395 to about 47% nickel, from about1% to about 5.1% molybdenum and an amount of nitrogen in accordance withthe following equation:
 18. The alloy of claim 1 further consistingessentially of said nitrogen being present in an amount from about 0.01%to about 0.15%.
 19. A lead frame comprising the alloy of claim 1.